1. Field of the Invention
The present invention relates to multi-wavelength light detecting and/or emitting apparatuses having serially arranged grating directioal couplers to be utilized in wavelength or frequency division multiplexing optical communication systems, wavelength or frequency division multiplexing optical interconnections, optical operations, recordings and measurements utilizing light in a certain wavelength range and the like. Such light detecting and/or emitting apparatuses include a multi-wavelength light emitting apparatus having a plurality of serially arranged semiconductor lasers and an integrated structure for emitting lights of different wavelengths from a common emission end surface, a demultiplexing light detecting apparatus having a plurality of serially arranged photodetector and so forth.
2Related Background Art
In general, in optical fiber communication systems and the like, a so-called demultiplexing photodetector is important for separating and detecting wavelength-multiplexed light signals per each wavelength. A prior art demultiplexing light detecting apparatus or photodetector, such as illustrated in FIG. 1, for example, exists (see Electronics Information Communication Society Autumn Grand Meeting Informal Paper B-469 (1989)). In this apparatus, an input light 1 having a plurality of wavelength components is reflected and diffracted by a Fourier diffraction grating 2, and is again imaged by a collimating lens 3 for the separation of the wavelength components. An optical fiber array 4 is disposed at the image position of the reflection-diffracted light, and each wavelength component is detected.
This apparatus, however, has the drawback that the entire apparatus inevitably becomes large for the apparatus must spatially expand the light. Furthermore, the optical path length of the light beam 1 needs to be prolonged in order to improve the wavelength resolution and reduce an cross talk, which also leads to the apparatus of a large size.
To solve these drawbacks, such an apparatus as is disclosed in FIG. 2 has been proposed (see Japanese Patent Laid-Open No. 2-4209). In this apparatus, on a substrate 11, there are provided an n-AlGaAs first reflection layer 12, an n-AlGaAs waveguide layer 13, an n-AlGaAs second reflection layer 14, an undoped GaAs/AlGaAs superlattice light absorption layer 15, diffraction gratings 16, an n-AlGaAs third reflection layer 17, a p-AlGaAs third reflection layer 18, three electrodes 1, 2 and 3 formed on the p-AlGaAs third reflection layer 18 and a common electrode 19 formed on the bottom of the substrate 11. As shown in FIG. 2, a pin photodiode is constructed by the p-A1GaAs third reflection layer 18, the undoped GaAs/AlGaAs superlattice light absorption layer 15, the n-AlGaAs second reflection layer 14, the electrodes 1, 2 and 3, the electrode 19 and so forth, and the grating 16 is formed therein. In this structure, out of an input light of wavelengths (.lambda..sub.1 +.lambda..sub.2 +.lambda..sub.3 +.lambda..sub.4) introduced into the n-AlGaAs waveguide layer 13, components of predetermined wavelengths (in this case, from the light input side, the components of .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 are transferred in this order by controlling respective voltages V.sub.1, V.sub.2 and V.sub.3) are transferred into the light absorption layer 17 by the respective gratings 16 to perform a wavelength selection function. Thus, the apparatus has been made compact in size, and the integration is facilitated.
In this apparatus, however, since the selected wavelength is tuned by the control of the voltage applied to each of the electrodes 1, 2, and 3, different voltages are necessarily applied to the respective photodetectors composed of the pin structures under the electrodes 1, 2 and 3, and hence an equal voltage cannot be applied thereto. Therefore, the problem occurs that the detected current I(.lambda..sub.2), I(.lambda..sub.3) and I(.lambda..sub.4) and response speed (i.e., sensitivity speed) differ among the respective photodetectors. In general, an optimal use voltage is given according to the structure of each photodetector.
On the other hand, a multi-wavelength light emitting apparatus or semiconductor laser is also a critical device for the wavelength division multiplexing optical transmission systems, optical interconnection systems and the like. In these optical transmission and interconnection systems and the like, it is needed to multiplex and transmit output lights from the multi-wavelength light emitting apparatus or a plurality of laser lights of different wavelengths.
Conventionally, this kind of multi-wavelength light emitting apparatus or semiconductor laser is fabricated by parallelly integrating semiconductor lasers having outputs of different wavelengths and arranging a structure for combining the output light signals. For example, there has been proposed a structure in which output lights from parallelly integrated semiconductor lasers are combined by a Y-type combining device and an output of multiplexed laser lights is emitted from a single outlet, as shown in FIG. 3 (see Japanese Patent Laid-Open No. 55-163888) and in FIG. 4 (see Japanese Patent Laid-Open No. 58-175884). In FIG. 3, lights radiated from distributed Bragg reflection (DBR) type semiconductor lasers 21a.about.21d provided with gratings 22a.about.22d are guided into a common light waveguide 23 after they propagate through respective light waveguides 24a.about.24d. The thus combined light is emitted from a single outlet. In FIG. 4, lights oscillated by plural lasers 31 (wavelengths .lambda..sub.1 .about..lambda..sub.10) are guided into a common light waveguide 32 after they propagate through respective light waveguides, and the thus combined light is emitted from a single outlet.
Furthermore, in another prior art apparatus illustrated in FIG. 5 (see Japanese Patent Laid-Open No. 62-229105), respective light waveguides 41a.about.41d are extended from respective light emission end surfaces 42 of plural semiconductor lasers 43 to wavelength selection type directional couplers 44 provided with respective gratings or periodic structures 45. The radiated lights are thus guided into a common light waveguide 46. Antireflection coats are formed on the respective light emission end surfaces 42, and no antireflection coats are formed on the other light emission end surfaces 48. The light waveguides 41a.about.41d are formed on a light waveguide substrate 49. There are further provided a light output fiber 50, and a high-reflection surface 51 is formed on the end surface of the fiber 50.
In such structures in which the semiconductor lasers are parallelly integrated and the combining portion for the output light is constructed by the curved light waveguides, however, the problem occurs that the size of the entire multi-wavelength semiconductor laser becomes extremely large. For example, assuming that the distance between the parallel semiconductor lasers is 100 .mu.m, at least 1 mm is needed in a lateral direction (direction of parallel arrangement) when ten lasers are integrated. And a coupler length of 30 mm is needed to combine two output lights which are remote from each other by a maximum of 1 mm, when a combining angle is assumed to be 2 degrees.
Thus, only a multi-wavelength semiconductor laser of a quite large size could be obtained according to the prior art structure. Furthermore, in the prior art combining device/multiplexing device, since the propagation length is long, magnitudes of the propagation loss and radiation loss at the curved portion are not negligible and the output light intensity of the laser light is reduced.